My favourite line in the film A Fish Called Wanda comes when Otto (Kevin Kline), a psychopathically idiotic ex-CIA operative, objects to being called "stupid" by Wanda (Jamie Lee Curtis). To which Wanda replies: "To call you stupid would be an insult to stupid people! I've worn dresses with higher IQs."

Until relatively recently, "dresses" could have been replaced by "mobile phones" in the script, and the line would still have raised a laugh. But that's changing fast. Quite how fast was revealed last week in an extraordinary report from the Pew Internet and American Life Project. This is the most sustained and intensive ongoing effort we have to chronicle the penetration of the internet into everyday life, and although it only surveys the US, many of its findings seem to be echoed in other industrialised countries, including ours.

The Pew report found that 35% of American adults now own a "smartphone", that is to say a mobile phone with a significantly more powerful processor and much better internet connectivity than an old-style handset which could do voice and text and not much else. Smartphones (think iPhone, Android, BlackBerry, Windows Mobile) usually also function as portable media players and cameras and have GPS navigation, Wi-Fi and mobile broadband access built in. Smartphone penetration seems to be following a similar pattern in the UK. A few months ago, a survey conducted by Olswang, a law firm specialising in the technology and media sectors, found that 22% of UK consumers already have a smartphone, with this percentage rising to 31% among 24- to 35-year-olds.

The first and obvious inference from these numbers is that the penetration of smartphones is rising more quickly than had originally been forecast. Who would have thought that such expensive devices would become so popular so quickly? Certainly not Nokia, a former giant of the mobile phone business, now in intensive care. But there's an even more significant discovery buried in the Pew survey results. It turns out that nearly 90% of smartphone owners access the internet or email on their phones, and two thirds of them do so on a typical day. A quarter of smartphone owners say that they now use their phones rather than PCs to go online. While many of these individuals have online access at home, Pew reports that roughly one third of them no longer bother with a high-speed home broadband connection.

What does this mean? Essentially, that we are on the slippery slope towards a much more controlled, less open, internet. If these trends continue, then it won't be all that long before a significant proportion of the world's internet users will access the network, not via freely programmable PCs connected via landline networks, but through tethered, non-programmable information appliances (smartphones) hooked up to tightly controlled and regulated mobile networks. And if that happens then the world will have kissed goodbye to the internet's revolutionary potential.

What makes the internet special is that it is a magical enabler of what the Stanford scholar Barbara van Schewick calls "permissionless innovation". If you're bright and have a good idea that can be implemented via software, then the internet will run it for you, with no questions asked and with very low entry barriers. At the moment, there are no gatekeepers who can keep out an innovator, no incumbents who can impose a swingeing tax on an innovative idea. But an internet accessed mainly via smartphones would be a very different kind of space – dominated by giant companies determined to repel newcomers, to protect obsolete business models and ensure that innovation happens at a pace determined by them rather than by the possibilities of technology and human ingenuity.

The danger, in other words, is that we move from an internet designed for people to a networked tailored only to the needs of corporations. This will be news, incidentally, to the UK's Department of Culture, Media and Sport, which – as the Open Rights Group pointed out last week – appears to think that the only role for government is to enable commercial exploitation of the net.

In a striking TED talk given recently in Edinburgh, the internet scholar Rebecca MacKinnon vividly spelled out some of the implications of this mind-set, and argued that combating it will require concerted citizen activism akin to the long-term campaigns that have begun to force western corporations to stop exploiting developing nations' child labour. She's right: but that means it'll be a long haul. And it needs to start now.